1. Field of the Invention
This invention relates to an electron beam control system for manually controlling an electron beam in an electron-beam lithographic system or the like.
2. Description of the Prior Art
An electron beam control system is adapted to adjust the currents of deflecting coils for deflecting an electron beam in x-axial and y-axial directions. The coil current is produced by, for example, converting a control signal given as a digital quantity into an analog signal by means of a digital-to-analog converter (hereinbelow, abbreviated to "DAC"). The adjustment of the coil current is therefore made by varying the digital input signal of the DAC.
In order to manually vary the digital input signal, the following methods have heretofore been practised:
(1) A set value is varied with digital switches.
This is a method in which the digital switches are disposed for the respective bits of the digital input signal for each deflecting coil, and the respective switches are manually turned "on" and "off", thereby to vary the set value. The controllability of the method is very inferior.
(2) A set value is varied on the basis of the output of a counter which counts the output pulses of an oscillator.
This is a method in which the frequency of the oscillator output pulses to be counted by the counter which has a length corresponding to the number of bits of the digital input signal is coarsely adjusted and finely adjusted, thereby to vary the set value. It requires a coarse adjustment switch as well as a fine adjustment switch for the oscillation frequency and an up/down change-over switch for the counter, in addition to an oscillation starting switch. Further, since the content of the counter vanishes at the turning-off of a power supply, readjustments are necessary when the power supply is closed again. Therefore, the controllability is inferior as in the foregoing method.